Integrated circuits are produced by photolithographic projection of patterns onto semiconductor wafers. For this purpose, layers provided with different electrical properties are usually applied on semiconductor wafers and in each case patterned lithographically. A lithographic patterning step may consist in applying a photosensitive resist, exposing the latter with a desired structure for the relevant layer and developing it, and subsequently transferring the resist mask thus produced into the underlying layer in an etching.
Dense line-space patterns such as those that are formed in the field of production of dynamic random access memories (DRAM) include structure elements having line widths of 110 nm or less, for example, in the region of the memory cell arrays.
Exposure devices are used in the field of semiconductor fabrication in order to form a pattern of structure elements in a photosensitive resist by lithographic projection on a semiconductor wafer coated with said resist. In this case, the choice of the lateral extent of the structure elements to be formed on the semiconductor wafer is restricted due to a lower resolution limit which is predetermined in particular by the exposure device. The resolution limit depends on many factors and is usually described in accordance with the following formula:bmin=k1*λ/NA. 
In this equation, λ represents the wavelength of the light source of the projection apparatus, NA represents the numerical aperture and k1 represents a factor dependent on various contributions such as, by way of example, the type of illumination, the resist layer used, the focus conditions and further parameters. In order to increase the resolution capability of the projection apparatus, three possibilities thus exist, in principle, these possibilities being discussed briefly below.
One possibility for increasing the resolution capability consists in reducing the exposure wavelength λ. Present-day projection apparatuses for photolithography use an exposure wavelength of 193 nm, for example. Efforts are currently being made in the art to reduce the exposure wavelength to 157 nm. However, exposure systems with such a short wavelength are associated with some technical problems.
The resolution limit of a projection apparatus can also be reduced by using modern lithographic techniques in the case of the masks used for the exposure. This relates first of all to the field of phase masks, which are also called phase shift masks. Moreover, different exposure modes are implemented, such as, by way of example, oblique illumination, quadrupol illumination or annular illumination, which likewise bring about an improvement in the resolution capability of the projection apparatus. These types of illumination are also referred to as OAI illumination (off-axis illumination) in the art. In contrast to perpendicularly incident illumination, significantly more higher orders of diffraction are transferred in the projection objective in the case of oblique illumination.
The so-called RET (resolution enhancement technique) methods are known as a further possibility; in these methods the structure elements on the mask often also contain, alongside the circuit patterns to be imaged, further elements that improve the resolution of the projection apparatus. Alongside the elements known in the art for an optical proximity correction (OPC) provision is also made for using structure elements lying below the resolution limit in the vicinity of structure elements to be formed.
These techniques, individually or in combination, enable the resolution capability of a projection apparatus to be significantly improved. It must be assumed, however, that at the currently prevailing exposure wavelength of 193 nm, the improvement possibilities can no longer be exhausted to an extent such that it would be possible, by way of example, to effect patterning with very small resolutions of 50 nm. The resolution capability can also be increased, however, if the numerical aperture NA is increased.
This is exploited in the case of immersion lithography, for example, in which the light of the projection apparatus is transmitted from the projection objective onto the resist layer not in air vacuum but rather within an immersion liquid (for example water). It is thus possible to retain values for the numerical aperture which are greater than 1. Together with a k1 factor of about 0.3, it would thus be possible, at an exposure wavelength of 193 nm, to obtain a resolution capability of 50 nm without having to switch to the exposure wavelength of 157 nm, which is technologically problematic at the present time.
A problem to which little consideration has been given hitherto in this context is that the immersion liquid also influences the optical transmission. The immersion liquid is in direct contact with the resist layer or with a covering layer (so-called top coating) applied above the resist layer. During the exposure process, in the first place chemical actions are initiated in the resist layer, but in addition gases are also liberated and may escape from the resist layer. The high-energy short-wavelength exposure light also provides locally for heating of the resist layer and thus also of the immersion liquid. However, the immersion liquid likewise contributes to the imaging quality.